A braiding machine enables a composite material part to be fabricated by braiding around a mandrel and over the whole of its length one or more layers of reinforcing fibers superposed one on the other.
Once the various layers have been formed, the assembly constituted by the mandrel and the layers that it is carrying is placed in a mold to inject resin into these layers. This resin is then polymerized, for example by heating, to constitute a raw rigid part.
Such a braiding machine 1, which is represented in FIG. 1, essentially includes a ring 2 lying in a vertical plane, the revolution axis AX of this ring thus being horizontal. This ring 2 carries a set of spools of reinforcing fibers 3 that converge toward a point or a region situated on the axis AX and in front of the plane of the ring. These fibers thus conjointly define a generally conical shape.
When the braiding cycle is started, the mandrel 4 is moved along the axis AX to pass through the ring 2 beyond the point of convergence of the fibers. At the same time, the spools carried on the ring 2 by motorized mobile supports are actuated to fabricate a sock of reinforcing fibers on the external face of this mandrel 4.
This sock covers the mandrel over the whole of its length once it has passed completely through the ring, i.e. once it is situated beyond the point of convergence of the fibers, which is itself offset relative to the ring.
The layer of reinforcing fibers is then cut downstream of the mandrel and the mandrel is demounted and then replaced behind the ring, in order to pass through it again for the formation of a second layer of reinforcing fibers radially superposed on the first.
In practice, the mandrel has its downstream end rigidly fastened to a rear rod and its upstream end rigidly fastened to a front rod by means of which it is pulled through the ring. The mandrel may to this end include at each of its ends a threaded hole, each rod then having a corresponding threaded end that is screwed into this threaded hole.
In operation, the layer of braided fibers surrounds the front rod, and is formed around the mandrel as the latter is pulled forward along the axis AX by this rod.
When this layer has been completely formed, a cord is passed through the braid, downstream of the mandrel, and this cord is tensioned parallel to the axis AX to maintain the point of convergence of the fibers in front of the ring and approximately on the axis AX.
The reinforcing fiber braid may then be cut between the mandrel and the region in which the cord passes through it. After this step, the front rod is unscrewed from the front end of the mandrel and the mandrel is unscrewed from the front end of the rear rod and extracted with the layer of reinforcing fibers that it is carrying.
The mandrel with the layer of fibers is then installed behind the ring again. The rear end of the rod that passes through the ring is then screwed into the front end of the mandrel and this rod, which was the rear rod in the previous step, then becomes the front rod.
Another rod is screwed to the rear end of the mandrel. These rods that are fastened to the mandrel are moreover retained in position on the axis AX by means of a plurality of bearings spaced from each other along the axis AX.
In practice, when the point of convergence of the fibers is held by the cord during removal and reinsertion of the mandrel, the position of this point of convergence is insufficiently well controlled to be sure that it remains centered on the axis AX, with the result that this point of convergence is offset radially relative to the axis AX.
Accordingly, when the mandrel is again installed in the machine and its front end is brought to bear against the area of convergence of the fibers, the radial offset of this area of convergence results in localized disorganization of the reinforcing fibers at the level of the end of the mandrel. The mechanical strength of the raw part produced is then significantly penalized at the level of this end.
One way to prevent this problem consists in operating the braiding machine so as to recenter the area of convergence of the carbon fibers before proceeding to remount the mandrel. However, this solution is costly in terms of fabrication time and increases the length of reinforcing fibers necessary for braiding each layer.